Nanoscale corundum powders, sintered compacts produced from these powders and method for producing the same

ABSTRACT

Nanoscale corundum powders are obtained by first producing an Al 2 O 3  precursor by adding seed crystals to an aqueous solution of an aluminium compound and adding a base and then converting the Al 2 O 3  precursor into corundum by calcination at a high temperature. Before the calcination, the salts that are present in addition to the Al 2 O 3  precursor are separated off. The resulting product is calcined at temperatures of 700 to 975° C. and any fines that may be present are removed. The resulting corundum powders can be sintered at temperatures of ≦1200° C. to produce compacts or components of multiple layer systems.

[0001] The invention relates to nanosize corundum (α-alumina) powders,to a process for preparing them and to their processing to producesintered bodies.

[0002] Pulverulent corundum (α-Al₂O₃) is an important raw material forproducing aluminium oxide ceramics, which can in principle be achievedin two ways. One way starts out from shaped bodies which have been madedirectly from α-alumina powder (α-Al₂O₃ powder), while the other startsout from shaped bodies which comprise a α-Al₂O₃ precursor (for examplethe γ or θ phase) which is then converted in situ into the α-Al₂O₃phase.

[0003] In the processing of customary α-alumina powders to give densesintered bodies, the sintering temperature of α-alumina is, depending onthe initial particle size used, from 1300 to 1600° C. There have been,many attempts to reduce the sintering temperature of α-alumina or toobtain the thermodynamically stable α-Al₂O₃ phase at significantly lowertemperatures. The most important obstacle in this context is the highactivation energy of homogeneous nucleation, which is greatly delayedkinetically so that such nucleation can only be achieved from the otherAl₂O₃ phases (e.g. the γ or θ phase) at relatively high temperatures,since the diffusion coefficients are higher here. There have thereforebeen many attempts to achieve a significant reduction in thetransformation temperature by addition of nuclei; cf. EP-A-554908, U.S.Pat. No. 4,657,754 and WO 98/55400.

[0004] For example, U.S. Pat. No. 4,657,754 describes nanosize α-aluminahaving a mean particle size of from 20 to 50 nm (“nanocorundum”) whichis prepared by seeding, and this allows the synthesis temperature to bereduced sufficiently for α-Al₂O₃ powder having a density of 3.78 g/cm³(corresponding to 95% of the theoretical density) to be present at 1000°C.

[0005] In CN-A-1085187, Weng et al. describe another process startingout from salt solutions, which likewise gives nanocorundum having adiameter of from 10 to 15 nm at synthesis temperatures of from 1100 to1300° C.

[0006] However, a synthesis temperature of 1000° C. is too high for manypurposes, in particular for cofiring processes in microelectronics usingfoils or the sintering of pastes to a high density. The same applies tothe relatively high sintering temperature of conventional α-aluminapowder of from 1300 to 1600° C.

[0007] It has now surprisingly been found that the synthesis temperaturecan be reduced to values below 1000° C. by means of a particular processtechnique which gives an only weakly agglomerated nanosize α-alumina(corundum) powder which can be sintered at relatively low sinteringtemperatures. This apparently small improvement is of great industrialimportance since a significantly wider field of application can beaddressed as a result. For example, novel multilayer systems can beprocessed in a single cofiring step, for which a plurality of sinteringsteps at relatively high temperatures were necessary in the past, sinceall multilayer elements present can be densified at the lower sinteringtemperature.

[0008] The invention provides a process for preparing nanosize α-aluminapowders, in which an Al₂O₃ precursor is firstly prepared by admixing anaqueous solution of an aluminium compound with seed crystals and addinga base and then converting the Al₂O₃ precursor into α-alumia bycalcination at elevated temperature, which is characterized in that thesalts present in addition to the Al₂O₃ precursor are separated off priorto the calcination, the product obtained is calcined at temperatures offrom 700 to 975° C. and any fines (<40 nm) present are removed.

[0009] Alutmruum compounds suitable for preparing the Al₂O₃ precursorare preferably water-soluble alurninium salts such as aluminium(III)mntrate, aluminium(III) chloride, aluminium(III) acetate oraluminium(III) ethoxide.

[0010] These aluminium compounds are, for example, dissolved indeionized water and admixed with seed crystals which preferably have aparticle size of <100 nm. Examples of suitable seed crystals areα-alumina or diaspore nuclei.

[0011] A base is added and, as a result, the desired Al₂O₃ precursorrequited for conversion into α-alumina at temperatures below 1000° C. isformed during a ripening time. Examples of bases which can be used areinorganic or organic bases such as sodium hydroxide, potassiumhydroxide, calcium hydroxide or magnesium hydroxide, ammonia, urea,aliphatic and aromatic amines, with particular preference being given tobases such as ammonia which can be separated off thermally.

[0012] The precipitation or ripening is usually carried out attemperatures of from 50 to 100° C., preferably from 70 to 90° C. andparticularly preferably from 80 to 90° C., over a period of from 20 to145 hours, preferably from 60 to 90 hours and particularly preferablyfrom 70 to 80 hours.

[0013] After this ripening, the preparation of nanocorundum ispreferably carried out by one of the following two alternative methods.

[0014] In method 1, the aqueous solvent is removed, preferably by freezedrying, and the salts present as impurities are decomposed thermally attemperatures of from 150 to 500° C., for example at 400° C. The productobtained is mechanically corninuted and converted into α-Al₂O₃ bycalcination at temperatures of from 700 to 975° C., preferably from 750to 950° C. and in particular from 800 to 900° C. Calcination is usuallycarried out for a period of from one to three hours.

[0015] The α-alumina powder obtained by method 1 has a high proportionof α-alumina, but as secondary phase still contains a small proportionof fines (<40 nm) which comprises mainly phases other than α-Al₂O₃. Itis important for the purposes of the invention to remove at least mostof these fines so that the nanosize α-alumina powder can subsequently bedensified at sintering temperatures of <1200° C.

[0016] The removal of the fines is preferably carried out bycentrifugation. For this purpose, the α-alumina powder which has beenprepared is dispersed in an aqueous solution with the aid of adispersant (surface modifier) and subsequently centrifuged one or moretimes. Suitable dispersants are, for example, mnorganic acids preferablyHNO₃), aromatic or aliphatic monocarboxylic, dicarboxylic orpolycarboxylic acids, aromatic or aliphatic oxacarboxylic acids such astrioxadecanoic acid (FODA), β-dicarbonyl compounds and amino acids. Thedispersant concentration is matched to the specific surface area of theα-alumina powder synthesized, so that, for example, 4-5 μmol ofdispersant are available per m² of Al₂O₃ surface.

[0017] In method 2, the salt loading is reduced or removed by dialysis.For this purpose, the solution containing the Al₂O₃ precursor isintroduced into dialysis tubes and the latter are placed in deionizedwater. The dialysed solution is subsequently frozen and freeze dried.The powder obtained can, if desired, be calcined at from 150 to 500° C.(e.g. 400° C.) to achieve complete removal of the salts still present.The powder is converted into α-Al₂O₃ as in method 1 by calcination attemperatures of from 700 to 975° C., preferably from 750 to 950° C. andin particular from 800 to 900° C.

[0018] In this method 2, no or very little fines comprising non-α-Al₂O₃phases are formed during the synthesis, so that the α-Al₂O₃ powderobtained can, after surface modification using suitable surfacemodifiers such as inorganic acids (preferably HNO₃), aromatic oraliphatic monocarboxylic, dicarboxylic or polycarboxylic acids, aromaticor aliphatic oxa carboxylic acids, e.g. trioxadecanoic acid (TODA),β-dicarbonyl compounds or amino acids, be densified directly atsintering temperatures of ≦1200° C. The amount of surface modifier ismatched to the specific surface area of the α-alumnina powdersynthesized, so that, for example, 4-5 μmol of dispersant are availableper m² of Al₂O₃ surface. The surface modification can be carried out,for example, by means of a ball mill (3-4 h, aluminium oxide millingmedia ≦1 mm), mortar mills, a three-roll mill or a kneader, matched tothe subsequent shaping technique.

[0019] The result is a redispersible α-alumina powder which can beprocessed further by various shaping processes such as tape casting,screen printing, pad printing, electrophoresis, slip casting, extrusion,injection moulding. The mean primary particle size is usually from 30 to150 nm, preferably from 40 to 100 nm and particularly preferably from 50to 70 nm. The α-alumina powder is only weakly agglomerated in theredispersed state. It has a phase purity (α-Al₂O₃ content) of >80% byweight, preferably >90% by weight and in particular >95% by weight, anda density of >3.90 g/cm³, preferably >3.93 g/cm³, particularlypreferably >3.95 g/cm³.

[0020] The α-alumina powder prepared according to the invention is mixedwith customary processing aids, e.g. organic solvents, binders,plasticizers, for further shaping. Suitable solvents are, for example,ethylene glycol, diethylene glycol monobutyl ether and diethylene glycolmonoethyl ether, either individually or as mixtures. Examples of binderswhich could be used are cellulose derivatives such ashydroxypropylcellulose, polyvinylpyrrolidone, acrylate polymers andoligomers, methacrylates such as tetraethylene glycol dimethacrylate andpolyethylene glycol dimethacrylate. Use is made of, for example, 15% byweight of binders, based on the solid employed. Plasticizers used are,for example, polyethylene glycol dimnethacrylates, polyethylene glycols(e.g. PEG 600, PEG 800, PEG 1000, PEG 2000, PEG 6000). Use is made of,for example, 25% by weight, based on the binder employed.

[0021] The nanosize α-alumina powders of the invention are suitable forproducing dense Al₂O₃ sintered bodies in the form of components orconstituents of multilayer structures. Specific applications of thesecomponents and multilayer systems are (micro)electronics, sensors (gas,pressure or piezoelectric sensors), ricrosystem technology (e.g.microreactors), ceramic filter elements and catalyst supports.

[0022] The following examples illustrate the invention.

EXAMPLE 1 Preparation of Nanocorundum

[0023] 16 l of deionized water are placed in a stirred glass vessel, and4 kg of A(NO₃)₃. 6H₂O while stirring. 60 g of aluminium oxide nucleiα-aluminium oxide or diaspore) are then added in the form of a 5-20% byweight aqueous suspension (pH>3). The mixture is heated to a temperatureof 85° C.±5° C. The pH of the solution is adjusted to 4.8±0.1 by meansof aqueous ammonia solution (25% by weight). The mixture is maintainedat a temperature of 85° C.±5° C. for 72 hours while stirring. After 72hours, two alternative ways of preparing nanosize α-alumnina can beemployed.

[0024] Method 1

[0025] The “solution” obtained is frozen (for example at −30° C.) andsubsequently dried (freeze drying). The powder is then heated to 400° C.at a heating rate of 2 K/min (air atmosphere) and maintained at thistemperature for one hour. After cooling, the powder is dry milled in amortar mill for one hour. The powder is subsequendy brought to 800° C.at a heating rate of 10 K/nun and immediately heated to 900° C. at aheating rate of 2 K/min and maintained at this temperature for one hour.The powder prepared in this way has a specific surface area of about20-60 m²/g and a density of 3.6-3.9 g/cm³, in each case depending on thenuclei used.

[0026] After cooling, the powder is dispersed in a ball mill usingaluminium oxide milling media (≦1 mm) and an organic acid (TODA) asdispersant/surface modifier for 3 hours. The dispersant content ismatched to the specific surface area of the aluminium oxide powdersynthesized, so that 4-5 μmol of TODA are present per m² of Al₂O₃surface. After the tiling process, the fines in the aluminium oxidepowder obtained are separated off by multiple centrifugation. Theseparation limit in the centrifiugation is, on the basis of calculation,at a particle size of about 40 nm. The fines comprise predominantly(>90%) non-α-Al₂O₃ particles. The centrifugate is freeze dried to removethe solvent.

[0027] Method 2

[0028] The “solution” obtained is purified by dialysis in portionscontaining about 400 g of ammonium nitrate to remove the dissolvedammonium nitrate ions. For this purpose, the solution is introduced intoa dialysis tube (pore size: 2.5-3 nm) and stored in deionized water forabout 2 hours, after which the water is replaced and dialysis iscontinued for another two hours. The dialysed solution is frozen (forexample at −30° C.) and subsequently dried (freeze drying). If desired,the powder can additionally be heated to 400° C. at a heating rate of 2K/min (air atmosphere) and maintained at this temperature for one hour.However, this step is not absolutely necessary. The powder issubsequently brought to 800° C. at a heating rate of 10 K/min andimmediately heated to 900° C. at a heating rate of 2 K/min. A holdperiod of one hour is employed at 900° C.

[0029] The powder prepared in this way has a specific surface area ofabout 18-22 m²/g and a density of 3.95-3.98 g/cm³. The primary particlesize is 40-70 nm, and the powder is weakly agglomerated in theredispersed state.

EXAMPLE 2 Production of Sintered Aluminium Oxide Layers in MultilayerSystems

[0030] 10.5 g of the α-Al₂O₃ prepared in Example 1 are homogeneouslymixed with 2.8 g of a 1:1 solvent mixture of ethylene glycol anddiethylene glycol monobutyl ether and 0.5 g of polyvinylpyrrolidone asbinder. As mixers, it is possible to use mortars, kneaders or mortarmills. The paste obtained is passed a number of times through athree-roll mill for final homogenization.

[0031] The aluminium oxide paste is applied by a thick layer method(screen printing) to previously sintered α-alumina substrates or green(unsintered) substrates comprising yttrium-stabilized (3 mol % of Y₂O₃)zirconium dioxide in dry layer thicknesses up to 30 μm and dried so thatthey were free of cracks at 80° C. in a convection drying oven. Theprinted layers on the α-alumina substrates are densified firmly at 1200°C. (heating rate: 5 K/min) with a hold time of one hour. Densificationof the α-Al₂O₃ layers printed onto green (unsintered) substratescomprising yttrium-stabilized zirconium dioxide is carried out in twostages. In the first stage, the organics present in the composite areremoved by thermal decomposition at temperatures up to 450° C. under aprotective gas atmosphere (nitrogen). The heating time is 10 hours, holdtime: 3 hours. Thermal densification to give the dense materialcomposite is carried out in an atmosphere furnace at temperatures of1200° C., hold time: 3 hours, heating rate: 5 K/min.

EXAMPLE 3 Production of Sintered Aluminium Oxide Bodies from α-Al₂O₃Powder Prepared According to the Invention

[0032] 2 g of the α-Al₂O₃ powder prepared in Example 1 are homogeneouslymixed with 1 g of a solvent mixture of ethylene glycol/diethylene glycolmonobutyl ether (1:1) and 0.15 g of a cellulose binder and dried at 100°C. 200 mg of the mixture are compacted in a uniaxial pressing toolhaving an internal diameter of 5 mm under a pressure of 200 MPa. Thecompact is subsequently after-compacted at 400 MPa in a cold isostaticpress. The pressed green body is densified thermally at 1200° C. (1 h)under an air atmosphere. After sintering, the shaped body has a densityof 3.85 g/cmt³ (96.5% of theory).

1. Process for preparing nanosize α-alumina powders, in which an Al₂O₃precursor is firstly prepared by admixing an aqueous solution of analuminium compound with seed crystals and adding a base and thenconverting the Al₂O₃ precursor into α-alumina by calcination at elevatedtemperature, characterized in that the salts present in addition to theAl₂O₃ precursor are separated off prior to the calcination, the productobtained is calcined at temperatures of from 700 to 975° C. and anyfines (<40 nm) present are removed.
 2. Process according to claim 1,characterized in that the aluminium compound used is aluminium(III)nitrate, aluminium(III) chloride, aluminium(II) acetate oraluminium(III) ethoxide.
 3. Process according to claim 1 or 2,characterized in that the seed crystals used are corundum or diasporenuclei.
 4. Process according to any of claims 1 to 3, characterized inthat a base which can be separated off thermally, preferably ammonia, isused for preparing the Al₂O₃ precursor.
 5. Process according to any ofclaims 1 to 4, characterized in that ripening at temperatures in therange from 50 to 100° C. is carried out during and/or after preparationof the Al₂O₃ precursor.
 6. Process according to any of claims 1 to 5,characterized in that the salts are separated off by dialysis and/orthermal decomposition prior to calcination.
 7. Process according to anyof claims 1 to 6, characterized in that the calcination is carried outat from 800 to 900° C.
 8. Process according to any of claims 1 to 7,characterized in that fines present are removed by dispersion of theα-alumina powder and subsequent centrifugation.
 9. Nanosize α-aluminapowder having an α-Al₂O₃ content of at least 80% by weight and a densityof at least 3.90 g/cm³, obtainable by the process of any of claims 1 to8.
 10. Compositions containing a nanosize α-alumina powder according toclaim 9 and customary processing aids.
 11. Process for producing denseAl₂O₃ sintered bodies, characterized in that a composition according toclaim 10 is shaped in a customary shaping process to give a shaped bodyor a constituent of a multilayer structure and is then sintered. 12.Process according to claim 11, characterized in that sintering iscarried out at temperatures of ≦1200° C.